Devices and methods for nerve mapping

ABSTRACT

One embodiment includes a method for monitoring nerve tissue which includes inserting a dilator into muscle, the dilator including first and second electrodes at the distal tip. While the dilator is in muscle, a system may communicate (a) a first series of unequal current amplitude applications (e.g., a series including 0.5, 7, 3, 5 mA applications) to the first electrode to produce at least a first evoked potential (e.g., a MAP or NAP), and (b) a second series of unequal current amplitude applications to the second electrode to produce at least a second evoked potential. The method further includes sensing the first and second evoked potentials and determining a relative location of a nerve based on the sensing of at least one of the first and second evoked potentials.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Nonprovisional patentapplication Ser. No. 12/770,923 filed on Apr. 30, 2010 and entitled“Devices and Methods for Nerve Mapping”, the content of which is herebyincorporated by reference.

BACKGROUND

As described in “An Anatomic Study of the Lumbar Plexus with Respect toRetroperitoneal Endoscopic Surgery”, SPINE Vol. 28, Number 5, pp.423-428, 2003, by Takatomo Moro et al., nerve mapping can be criticalduring surgery, such as endoscopic surgeries, to treat various lumbarspine diseases. For example, retroperitoneal endoscopic surgery has beenapplied to anterior interbody fusion for disc herniation, anteriordecompression and interbody fusion for burst fracture and discectomy forextreme lateral disc herniation. However, to perform such a surgery thepsoas major muscle may be separated. Doing so generates a risk of injuryto the lumbar plexus or nerve roots. Accurate nerve mapping is thuscritical to avoid trauma to nerves, such as those exposed duringdissection of the psoas muscle.

Various methods exist for monitoring nerves (i.e., nerve mapping) duringsurgical procedures. Such methods may determine when an electrifiedinstrument is approaching a nerve. Specifically, a known current iscommunicated to the instrument. As the instrument is manipulated withinor on the patient, the current may evoke a muscular response. Themuscular response is recorded and an auditory and/or visual signal isproduced which alerts the user, such as a physician, that the instrumentis considered to be near the nerve coupled to the responsive muscle.

However, such methods do not provide the user with the location of thenerve evoking the muscular response. Instead, they only suggest a nerveis in the vicinity of the instrument. Also, to record the muscularresponse the responsive muscle must not be numbed or temporarilyparalyzed with, for example, neuromuscular blockade agents. As a result,the user may have difficulty advancing the instrument through a musclethat is not only active (i.e., not paralyzed), but which may even becontracting to resist or fight the user's efforts.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the present invention will become apparentfrom the appended claims, the following detailed description of one ormore example embodiments, and the corresponding figures, in which:

FIGS. 1A-1D include a dilator in one embodiment of the invention.

FIG. 1E includes a stimulation and/or sensing system in an embodiment ofthe invention.

FIG. 2 includes a nested dilator system in an embodiment of theinvention.

FIGS. 3-5 include methods for nerve mapping in embodiments of theinvention.

FIG. 6 includes a system for operation with an embodiment of theinvention.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth.However, it is understood that embodiments of the invention may bepracticed without these specific details. Well-known circuits,structures, and techniques have not been shown in detail to avoidobscuring an understanding of this description. References to “oneembodiment”, “an embodiment”, “example embodiment”, “variousembodiments” and the like indicate the embodiment(s) so described mayinclude particular features, structures, or characteristics, but notevery embodiment necessarily includes the particular features,structures, or characteristics. Further, some embodiments may have some,all, or none of the features (utilized in the same or differingorientations and orders) described for other embodiments. Also, as usedherein “first”, “second”, “third” and the like describe a common objectand indicate that different instances of like objects are being referredto. Such adjectives are not intended to imply the objects so describedmust be in a given sequence, either temporally, spatially, in ranking,or in any other manner. The terms “coupled” and “connected,” along withtheir derivatives, are not intended as synonyms for each other. Rather,in particular embodiments, “connected” may be used to indicate that twoor more elements are in direct physical or electrical contact with eachother. “Coupled” may mean that two or more elements co-operate orinteract with each other, but they may or may not be in direct physicalor electrical contact.

FIGS. 1A-D include various renderings of dilator 100 in an embodiment ofthe invention. Dilator 100 may include electrodes 105, 110, 115, 120,125, 130, 135, and 140 that connect to a distal tip 103 of the device.As used herein “tip” constitutes a leading edge of device 100. Theelectrodes may indeed wrap around tip 103. As will be described morefully below, locating the electrodes most distally may ensure anyinitial forward advancing contact or approach to a nerve is identifiedat the earliest instance (as compared to a case where an electrode islocated proximal to leading edge 103). Embodiments of the invention arenot limited to having eight electrodes and may instead include one(e.g., unipolar), two, three, four, five, six, seven, nine, ten (and soon) electrodes. Each electrode may be dimensioned alike. Takingelectrode 105 for example, electrode 105 may have a 3 mm width 101.Other embodiments may include electrode with widths of 1, 2, 4, 5 mm(and so on). The electrodes may be spaced a distance 102, such as 8 mmfor example, from one another. The electrodes may have various shapesincluding circular, rectangular, square, oblong, and the like.

Ground electrode 145 is located on dilator 100. Ground 145 may belocated, for example, 30 mm proximal of the proximal edge or tip ofelectrode 105. In other embodiments ground electrode 145 may be located40 mm proximal from electrode 105 and still other embodiments maydistance the electrodes at 100 mm, 200 mm, 300 mm, and so on. Electrode145 may be formed as a 6 mm wide band that traces a perimeter of dilator100. Of course ground electrode 145 need not trace the entire perimeterof device 100 in other embodiments. For example, ground electrode 145may be located in one or more sections on device 100, none of whichconstitute a continuous ring that traces the entire perimeter of dilator100. Outer diameter 165 of dilator 100 may be 25 mm but otherembodiments are not so limited.

Device 100 may include recesses 150, 155 to allow components to snap fitonto device 100 with a “one way fit only” contact clip to be attached tothe dilator or a “coded” connector providing the interface between thehardware and stimulation and/or sensing system 195 (FIG. 1E). Dilator100 may have various shapes or cross-sections including circular,rectangular, square, oblong, and the like. Also, device 100 may includemembers 158, 168 that mate with differently sized dilators, guide wires,shims, and the like. Device 100 could be a disposable single useattachment to other dilators or devices.

FIG. 1E includes stimulation and/or sensing system 195 in an embodimentof the invention. Electrodes 143 may include electrodes 105, 110, 115,120, 125, 130, 135, 140, and 145 located on dilator 100. Dilator 100 andelectrodes 185 (e.g., surface patch and/or sub-dermal needle) bothcommunicate with unit 180. Unit 180 includes a current source 181 forcommunicating current to any of electrodes 143, 185; amplifier unit 182for amplifying signals from electrodes 143, 185; filter(s) 183 andgeneral signal processing circuitry, such as analog/digital anddigital/analog converter 184, for processing signals from electrodes143, 185. Unit 180 communicates with computer system 186 (FIG. 6). Theuse of system 195 is discussed further below.

FIG. 2 includes dilator 200 in an embodiment of the invention. Dilator200 is shown nested over progressively smaller dilators 205, 210, and215. Various embodiments may include dilators that nest over more orfewer dilators. Also, dilator 200 may nest within, over, or couple toother dilators or devices, such as probes, retractors, snips, cuttingdevices, rasps, trocars, spreaders, distracters, shims, scrapers,chisels, disc cutters, curettes, suction probes, tamps, and the like. Asystem may include one, two, three, four, five (and so on) nesteddilators. All of the dilators, or just some of the dilators, may includeelectrodes as described above (e.g., electrodes 105, 110, 115, 120, 125,130, 135, 140, 145).

Embodiments described herein, such as device 100, may be used in varioussurgical approaches. For example, dilator 100 may be used in variousretroperitoneal space procedures such as a lateral interbody fusion(LIF) procedure where the spine is approached from the side of thepatient. Lumbar interbody fusion may entail fusing vertebrae together toreduce their motion after possibly removing some or the entireintevertebral disc located between the fused vertebrae. An intevertebralspacer may replace the removed disc material. Approaching laterally andvia endoscopic means allows for potentially less trauma to muscles,ligaments, blood vessels, and/or organs as opposed to techniques such asposterior, transforaminal, or anterior interbody fusion. Approachinglaterally still provides good access to the spinal column such as may beneeded for fusion at lumbar levels L1 through L5. There are manyindications for LIF such as, for example, degenerative disc disease,recurrent disc herniation, post-laminectomy syndrome, adjacent levelsyndrome, degenerative spondylolisthesis, degenerative scoliosis, andposterior pseudarthrosis. Thus, retroperitoneal endoscopic surgery(e.g., LIF) may be applied to various spinal disorders.

However, such procedures may require the user to traverse muscles, suchas the psoas major muscle, in order to access the spine. When the psoasmuscle is separated during retroperitoneal endoscopic surgery, there isa potential risk of injury to the lumbar plexus or nerve roots. Thus, inan embodiment dilator 100 is inserted into psoas muscle to facilitatenerve mapping.

FIG. 3 includes a method 300 in an embodiment of the invention. In block305 a user inserts dilator 100 into muscle, such as the psoas muscle. Asthe user advances dilator 100 through the psoas muscle and towards thespinal column, care must be taken to detect neighboring nerves. Thus, inblock 310 current is communicated to dilator 100. Specifically, currentmay be supplied to any or all of electrodes 105, 110, 115, 120, 125,130, 135, and 140. Doing so may result in a muscle action potential(MAP) from the level of related muscle groups.

For example, in block 315 current is simultaneously supplied to distaltip electrodes 105, 110, 115, 120, 125, 130. These electrodes may bedesignated as cathodes. Other electrodes, such as electrodes 135, 140,may be designated as anodes. Current may be delivered to patient tissuevia electrodes 105, 110, 115, 120, 125, 130. For example, 10 mA from aConstant Current Stimulation (CCS) device may be supplied with astimulus rate of 9.7 cycles/second (Hz) at a duration of 200microseconds. Should one of the cathodes be functionally proximate to anerve the stimulation current may depolarize the nerve and produce anerve action potential (NAP) volley (block 320). The NAP may in turnproduce a MAP (block 325) in the muscle(s) related to the affectednerve. In other embodiments other electrodes (e.g., electrode 105), noelectrodes, electrodes located elsewhere on device 100 (proximal to end103 and located along shaft of device 100), or electrodes locatedelsewhere on the body (e.g., needle or surface electrode) may serve asanodes or references.

In an embodiment, in diamond 335 a determination is made regardingwhether a MAP is being sensed or recorded. The MAP may be recordedusing, for example, Disposable Sub-dermal Needles (DSNs) inserted intotissue (e.g., muscle, fat, facie, and the like) and/or surface patchelectrodes. For example, two DSNs may be inserted into or near one ormore of the following muscles including, without limitation, the vastuslateralis, gastrocnemius, anterior tibialis, abductor hallucis longusmuscles and the like. The MAP may be sensed from each or some of themonitored muscles. In order to create a MAP that can be measured, themonitored muscles should not be subject to muscle relaxants at a levelthat would inhibit detection of MAPs. Amplifier filters 183 may be setto allow signals at 30 Hz-2,000 Hz for each recording channel with 50microvolts/division for resolution.

If no MAP is sensed, in block 336 a determination is made that no nerveis nearby and device 100 may be moved within the muscle. For example,the device may be moved from a stationary position or continued along apath already being traversed. If a MAP is sensed, electrodes 120, 125,and 130 may be electronically disconnected from current using, forexample, a multiplexor. In block 340 current is then suppliedsimultaneously to only electrodes 105, 110, and 115 while electrodes135, 140 may be designated as anodes. Current may again be supplied at,for example, 10 mA. In diamond 345, a determination is made regardingwhether a MAP is sensed. For example, a determination may be maderegarding whether a MAP exceeding a threshold of 50 microvolts isdetected. If a MAP is sensed, in block 350 current may be supplied toelectrodes 105, 110, 115 in various ways of titration.

As one example of current titration, in block 350 0.5 mA is provided toelectrode 105, then to electrode 110, and then to electrode 115. Inother words, the current is supplied non-simultaneously to theseelectrodes. If no resultant MAP is detected resulting from any of the0.5 mA current applications, a higher current level (e.g., 7 mA) may besupplied sequentially (i.e., non-simultaneously) to each of electrodes105, 110, 115. If no MAP is detected based on the administration of 7mA, a current level higher than the immediately preceding 7 mA level(e.g., 8.5 mA) may be supplied sequentially to each of electrodes 105,110, 115. If a MAP is detected for only one of electrodes 105, 110, 115based on the administration of 7 mA, then the electrode that led to theMAP is determined to be closest to a nerve. However, if two or moreelectrodes, such as three electrodes 105, 110, 115, each produce a MAPbased on the administration of 7 mA, then a current level lower than theimmediately preceding current level (i.e., 7 mA) may be suppliedsequentially to each of target electrodes 105, 110, 115. For example, 5mA may be sequentially supplied to the three electrodes. The titratingprocess then continues for those three electrodes. If no resultant MAPis detected resulting from 5 mA current applications, a higher currentlevel (e.g., 6 mA) may be supplied sequentially to each of electrodes105, 110, 115. If a MAP is detected for only one of electrodes 105, 110,115 based on the administration of 5 mA, then the electrode that led tothe MAP is determined to be closest to a nerve. However, if two or moreelectrodes, such as all three electrodes, produce a MAP based on theadministration of 5 mA, then a current level lower than the immediatelypreceding current level (i.e., 5 mA) may be supplied sequentially toeach of target electrodes 105, 110, 115. For example, 3 mA may besequentially supplied to the three electrodes. The titrating processthen continues. On the other hand, if instead of three electrodesproducing a MAP based on the administration of 5 mA, only two electrodesproduce a MAP based on the administration of 5 mA, then a current levellower than the immediately preceding current level (i.e., 5 mA) may besupplied sequentially to only those two electrodes. For example, 3 mAmay be sequentially supplied to only the two electrodes at issue. Thetitrating process then continues.

As another example of current titration, in block 350 a sequentialseries of stimulation may be provided to electrode 105 starting at 0.5mA and increased in 0.25 mA steps until a MAP is detected and/or anupper threshold (e.g., 10 mA) is reached. When a MAP is first sensed,the corresponding current level is noted. This sequence is repeated forelectrodes 110 and 115. The contact point among electrodes 105, 110, 115closest to the nerve may be deemed to be the electrode or point thatneeds the least amount of current to produce a recorded or sensed MAP.

In diamond 345, if no MAP is sensed then in block 347 electrodes 105,110, and 115 are disconnected from current and current is communicatedto electrodes 120, 125, and 130. In diamond 348, a determination is madeto see if a MAP is detected. If not, the method proceeds to block 310for retesting. If a MAP is sensed based on current supplied toelectrodes 120, 125, and 130, then current may be supplied in a titratedform to those electrodes in a manner similar to block 350. For example,in block 349 0.5 mA is provided to electrode 120, then to electrode 125,and then to electrode 130. If no resultant MAP is detected resultingfrom any of the 0.5 mA current applications, a higher current level(e.g., 7 mA) may be supplied sequentially (i.e., non-simultaneously) toeach of electrodes 120, 125, and 130. If no MAP is detected based on theadministration of 7 mA, a current level higher than the immediatelypreceding 7 mA level (e.g., 8.5 mA) may be supplied sequentially to eachof electrodes 120, 125, and 130. If a MAP is detected for only one ofelectrodes 120, 125, and 130 based on the administration of 7 mA, thenthe electrode that led to the MAP is determined to be closest to anerve. However, if two or more electrodes, such as all three electrodes,produce a MAP based on the administration of 7 mA, then a current levellower than the immediately preceding current level (i.e., 7 mA) may besupplied sequentially to each of the target electrodes 120, 125, and130. For example, 5 mA may be sequentially supplied to the threeelectrodes. The titrating process then continues. If no resultant MAP isdetected resulting from 5 mA current applications, a higher currentlevel (e.g., 6 mA) may be supplied sequentially to each of electrodes120, 125, and 130. If a MAP is detected for only one of electrodes 120,125, and 130 based on the administration of 5 mA, then the electrodethat led to the MAP is determined to be closest to a nerve. However, iftwo or more electrodes, such as three electrodes, produce a MAP based onthe administration of 5 mA, then a current level lower than theimmediately preceding current level (i.e., 5 mA) may be suppliedsequentially to each of the target electrodes 120, 125, and 130. Forexample, 3 mA may be sequentially supplied to the three electrodes. Thetitrating process then continues.

As another example of current titration, in block 349 a sequentialseries of stimulation is provided to electrode 120 starting at 0.5 mAand increased in 0.25 mA steps until a MAP is detected and/or an upperthreshold (e.g., 10 mA) is reached. When a MAP is first sensed, thecurrent level is noted. This sequence is repeated for electrodes 125 and130. The contact point among electrodes 120, 125, and 130 closest to thenerve will be the electrode or point that needs the least amount ofcurrent to produce a recorded or sensed MAP.

Blocks 349 and 350 lead to block 352. In block 352, the nerve locationis indicated to the user. For example, the indication may include agraphical user interface (GUI) that indicates the relative nervelocation in relation to device 100 and/or to particular electrodes ondevice 100 (e.g., proximal, distal, anterior, posterior, inferior,superior, and the like). The user may then “steer device 100 away” fromthe electrode highlighted by the GUI as being problematically close to anerve. Auditory alarms (e.g., beep) and visual alarms (e.g., flashingred indicator) may be used to indicate grades of proximity to nerves(e.g., continuous beep and/or rapid blinking red indicator upon closerange of device 100 to a nerve). Such a feedback mechanism may beapplicable to any of the many embodiments disclosed herein.

In some embodiments titrated current may be supplied to all electrodessuch as, for example, electrodes 105, 110, 115, 120, 125, 130. In anembodiment method 300 is altered to move directly from block 335 “yes”path to non-simultaneously providing titrated current to each ofelectrodes 105, 110, 115, 120, 125, 130. In this embodiment as well asothers, titrated current may even be supplied to electrodes 135, 140 bydesignating other electrodes as anodes or references, even if only for atemporary period. As indicated in block 352, nerve location may still bedetermined based on which electrode had the lowest threshold forproducing a MAP. Also, embodiments do not necessarily requiredetermining which single electrode has the lowest threshold forproducing a MAP or NAP. For example, merely determining that any of twoor three electrodes (e.g., 105 and 110) has a lower threshold than otherelectrodes (e.g., 125, 130) may provide enough precision to still guidea user away from problematic areas. An embodiment may allow the user toconfigure the level of specificity he or she wants. Of course, thetitration sequencing may occur so quickly that there is littleappreciable burden associated with high specificity determination,thereby lowering the need to configure the system for lower specificity.This applies to other embodiments described herein, such as theembodiment of FIG. 5 discussed below.

Repeating the current titration while device 100 is not moved may serveto confirm the findings. System 195 may automatically repeat thesequence continually or the sequence of method 300, or modificationsthereof, may be repeated upon user command. Also, rotating or movingdevice 100 may also confirm the findings as the nerve should accordinglybe closest to another electrode depending on the movement of device 100(e.g. rotation of device 100). The method then returns to block 305 asthe user adjusts the dilator away from any impending nerves and movesdilator 100 to a spot on the spinal column, such as the annulus, wherethe fusion procedure may continue (e.g., nucleus and annulus partial orcomplete removal, spacer implant, and the like). Other devices (e.g.,tissue retractors) may be advanced over or through the dilator toprovide instrument access to the disc space.

FIG. 4 includes a method 400 in an embodiment of the invention. In block403 muscle relaxants may be administered to the patient. The relaxant,such as succinolcholine, vecuronium, and/or rocuronium, may be suppliedat a level such that the relaxant relaxes one or more of the followingmuscles including the iliocostalis thoracis, iliocostalis lumborum,spinalis thoracis, internal/external obliques, psoas, multifidus, andintertransversarii lumborum muscles and the like. As a result, the userdoes not need to fight against various muscles partially contracting, orcontracting at full force, during the procedure. In block 404 twosub-dermal needle electrodes (e.g., DSNs) are used. Two DSNs (e.g.,cathode and anode) are inserted close or functionally proximate to anerve, such as one or more of the following nerves: iliohypogastric,ilioinguinal, lateral femoral cutaneous, genitofemoral, femoral,obturator, sciatic, tibial, peroneal, lumbosacral plexus nerves and thelike.

In block 405 a user inserts dilator 100 into muscle, such as, forexample, the psoas muscle during a LIF procedure. As dilator 100advances through the psoas muscle care must be taken to detectneighboring nerves. Thus, in block 410 current, at a known energy levelsuch as 20, 25, 30, 35, 40, or 45 mA, is communicated to a distal nerve(e.g., sciatic or femoral nerves) via the DSN(s). The stimulus rate maybe 11.1 cycles/second (Hz) for a duration of 300 microseconds.Communicating the energy to the distal nerve(s) may result in a NAP(s)that is detected in one or more electrodes 105, 110, 115, 120, 125, 130,135, and 140. Sensing a NAP, instead of a MAP, may be required if themuscle relaxant given the patient is such that evoked potentials cannotbe monitored by sensing MAPs from the paralyzed muscles. Each electrodeof device 100 may communicate with a separate channel on a monitoringdevice. Electrodes 135 and 140 may be designated as referenceelectrodes, although in other embodiments electrode 105 is the solereference and in still other embodiments electrodes 105, 110, 115, 120(and so on) constitute multiple references. The remaining electrodes105, 110, 115, 120, 125, and 130 may be free to sense NAPs.

In diamond 415, a determination is made regarding whether any ofelectrodes 105, 110, 115, 120, 125, 130, 135, and 140 has detected a NAPresulting from the stimulus current. If no, in block 420 a determinationis made that no nerve is nearby and the user may further advance device100 from a static location or continue along trajectory already beingtraversed. However, in block 425 if a NAP is detected the electrodeclosest to the nerve will be the point that senses a NAP of the highestamplitude.

An indication may be given to the user as to which electrode recordedthe highest amplitude. The indication may include the aforementioned GUIthat indicates the nerve location in relation to an electrode of device100. Auditory and visual alarms may be used to indicate grades ofproximity to nerves.

By repeating, automatically or manually, the current applicationdescribed in block 410, while device 100 is not moved, may serve toconfirm the findings. Also, rotating or moving device 100 may alsoconfirm the findings as the nerve should accordingly be closest toanother electrode depending on the movement of device 100. The user maythen adjust dilator 100 away from any impending nerves and to a spot onthe spinal column where the fusion procedure may continue. Other devicesmay be advanced via (e.g., over or within) the dilator to provideinstrument access to the disc space.

FIG. 5 includes a method 500 in an embodiment of the invention. In block503 muscle relaxants may be administered to the patient to relax variousmuscles, such as those described above. In block 504 two DSNs areinserted close to a nerve, such as those discussed above. In block 505 auser inserts dilator 100 into muscle, such as the psoas muscle. As theuser advances dilator 100 through the psoas muscle and towards thespinal column, care must be taken to detect neighboring nerves. Thus, inblock 510 current, such as 10 mA, is communicated to dilator 100.Specifically, current may be supplied to any or all of electrodes 105,110, 115, 120, 125, 130, 135, and 140.

For example, in block 515 current is simultaneously supplied to distaltip electrodes 105, 110, 115, 120, 125, 130. These electrodes may bedesignated as cathodes. Other electrodes, such as electrodes 135, 140may be designated as anodes. Current may be delivered to patient tissuevia electrodes 105, 110, 115, 120, 125, 130 using known energy levels,such as 10, 20, 25, 30, 35, 40, or 45 mA, produced from a CCS device. Inthis example, 10 mA is used with a stimulus rate of 11.1 cycles/second(Hz) for a duration of 200 microseconds. Should one of the cathodes befunctionally proximate to a nerve the stimulation current may depolarizethe nerve and produce a NAP volley (block 520).

In diamond 535 a determination is made regarding whether a NAP is beingsensed or recorded. The NAP may be recorded using two DSNs located closeto nerves (e.g., located in fat, facie, muscle, and various othertissues) such as those tissues listed above. Amplifier filters 183 maybe set to allow signals at 30 Hz-2,000 Hz for each recording channelwith 50 microvolts/division for resolution.

If no NAP is sensed, in block 536 a determination is made that no nerveis nearby and device 100 may be moved within the muscle. However, if aNAP is sensed, electrodes 120, 125, and 130 may be disconnected fromcurrent. In block 540 current is then supplied simultaneously to onlyelectrodes 105, 110, and 115 while electrodes 135, 140 may be designatedas anodes. Current may again be supplied at 10 mA. In diamond 545, adetermination is made regarding whether a NAP is sensed. If a NAP issensed, in block 550 current may be supplied in a titrated form.

As one example of current titration, in block 550 0.5 mA is provided toelectrode 105, then to electrode 110, and then to electrode 115. Inother words, the current is supplied non-simultaneously to theseelectrodes. If no resultant NAP is detected resulting from any of the0.5 mA current applications, a higher current level (e.g., 7 mA) may besupplied sequentially (i.e., non-simultaneously) to each of electrodes105, 110, 115. If no NAP is detected based on the administration of 7mA, a current level higher than the immediately preceding 7 mA level(e.g., 8.5 mA) may be supplied sequentially to each of electrodes 105,110, 115. If a NAP is detected for only one of electrodes 105, 110, 115based on the administration of 7 mA, then the electrode that led to theNAP is determined to be closest to a nerve. However, if two or moreelectrodes, such as three electrodes, produce a NAP based on theadministration of 7 mA, then a current level lower than the immediatelypreceding current level (i.e., 7 mA) may be supplied sequentially toeach of the target electrodes 105, 110, 115. For example, 5 mA may besequentially supplied to the three electrodes. The titrating processthen continues. Other current titration methods are possible. Forexample, a sequential series of stimulation is provided to electrode 105starting at 0.5 mA and increased in 0.25 mA steps until a NAP isdetected and/or an upper threshold is reached. When a NAP is firstsensed, the current level is noted. This sequence is repeated forelectrodes 110 and 115. The contact point among electrodes 105, 110, 115closest to the nerve will be the electrode or point that needs the leastamount of current to produce a recorded or sensed NAP.

In diamond 545, if no NAP is sensed then in block 547 electrodes 105,110, and 115 are disconnected from current and current is communicatedto electrodes 120, 125, and 130. In diamond 548, a determination is madeto see if a NAP is detected. If not, the method proceeds to block 510for retesting. If a NAP is sensed based on current supplied toelectrodes 120, 125, and 130, then current may be supplied in a titratedform to those electrodes in a manner similar to block 550. For example,in block 549 0.5 mA is provided to electrode 120, then to electrode 125,and then to electrode 130. If no resultant NAP is detected resultingfrom any of the 0.5 mA current applications, a higher current level(e.g., 7 mA) may be supplied sequentially (i.e., non-simultaneously) toeach of electrodes 120, 125, and 130. If no NAP is detected based on theadministration of 7 mA, a current level higher than the immediatelypreceding 7 mA level (e.g., 8.5 mA) may be supplied sequentially to eachof electrodes 120, 125, and 130. If a NAP is detected for only one ofelectrodes 120, 125, and 130 based on the administration of 7 mA, thenthe electrode that led to the NAP is determined to be closest to anerve. However, if two or more electrodes, such as three electrodes,produce a NAP based on the administration of 7 mA, then a current levellower than the immediately preceding current level (i.e., 7 mA) may besupplied sequentially to each of the target electrodes 120, 125, and130. For example, 5 mA may be sequentially supplied to the threeelectrodes. The titrating process then continues. As another example ofcurrent titration, in block 549 a sequential series of stimulation isprovided to electrode 120 starting at 0.5 mA and increased in 0.25 mAsteps until a NAP is detected and/or an upper threshold is reached. Whena NAP is first sensed, the current level is noted. This sequence isrepeated for electrodes 125 and 130. The contact point among electrodes120, 125, and 130 closest to the nerve will be the electrode or pointthat needs the least amount of current to produce a recorded or sensedNAP.

Blocks 549 and 550 lead to block 552. In block 552, the nerve locationis indicated to the user using, for example, embodiments describedabove.

In some embodiments titrated current may be supplied to all electrodessuch as, for example, electrodes 105, 110, 115, 120, 125, 130. In anembodiment method 500 is altered to move directly from block 535 “yes”path to non-simultaneously providing titrated current to each ofelectrodes 105, 110, 115, 120, 125, 130. In this embodiment as well asothers, titrated current may even be supplied to electrodes 135, 140 bydesignating other electrodes as anodes or references, even if only for atemporary period. As indicated in block 552, nerve location may still bedetermined based on which electrode had the lowest threshold forproducing a NAP.

Repeating the current titration while device 100 is not moved may serveto confirm the findings. System 195 may automatically repeat thesequence continually or the sequence of method 500, or modificationsthereof, may be repeated upon user command. Also, rotating or movingdevice 100 may also confirm the findings as the nerve should accordinglybe closest to another electrode depending on the movement of device 100.The method then returns to block 505 as the user adjusts the dilatoraway from any impending nerves and moves dilator 100 to a spot on thespinal column, such as the annulus, where the fusion procedure maycontinue (e.g., nucleus and annulus partial or complete removal, spacerimplant, and the like). Other devices (e.g., tissue retractors) may beadvanced over the dilator to provide instrument access to the discspace.

Thus, using embodiments described herein may facilitate locating asafety zone for, as an example, the lumbar plexus and nerve roots whenperforming LIF or other procedures. These safety zones may be locatedat, for example, L2-L3 and above, between the cranial third of the L3vertebral body and L4-L5 (noting the genitofemoral nerve locatedanteriorly of the center of the vertebral body). Fusion or workregarding L1-L3 may be coupled with monitoring or stimulating thelateral femoral cutaneous nerve. Work regarding L4, L5, S1, or S2 may becoupled with monitoring or stimulating the sciatic nerve.

Thus, one embodiment includes a method for monitoring nerve tissue whichincludes inserting a dilator into muscle (e.g., psoas muscle during aLIF procedure), the dilator including a distal end portion includingfirst and second electrodes. The electrodes may be at the distal tip,wrap around the tip, or be located entirely proximal of the tip butstill located in the distal region of the dilator. While the dilator isin muscle, a system may communicate (a) a first series of unequalcurrent amplitude applications (e.g., a series including 0.5, 7, 3, 5 mAapplications) to the first electrode to produce at least a first evokedpotential (e.g., MAP or NAP), and (b) a second series of unequal currentamplitude applications to the second electrode to produce at least asecond evoked potential. The method may further include sensing thefirst and second evoked potentials and determining a relative locationof a nerve based on the sensing of at least one of the first and secondevoked potentials. In an embodiment, the relative location is relativeto one or more electrodes of the dilator. In an embodiment, if the firstpotential is produced using a lower current level than that whichproduced the second potential, the nerve may be determined to be closerto the first electrode. Embodiments that include communicating a seriesof unequal current amplitude applications to an electrode may includetitrating current to the electrode to determine a current amplitudeneeded to produce the first evoked potential. Titrating may be doneforwards, backwards, or combinations thereof. For example, titrating mayprogress up from a low current to a high current, from a high current toa low current, or between high and low currents to thereby box in aresponse of interest.

Embodiments described herein are not limited to dilators and may insteadbe implemented with devices and equipment such as, for example, probes,retractors, snips, cutting devices, rasps, trocars, spreaders,distracters, shims, scrapers, chisels, disc cutters, curettes, suctionprobes, tamps, and the like, which are inserted into innervated areas ofthe body that call for nerve mapping. For example, electrodes 105, 110,115, 120, 125, 130, 135, and 140 could be located along the leadingdistal edge of such devices, such as a probe. The probe could be used toconduct nerve mapping, as explained herein, before, after, orindependent of use of dilator 100. For example, the probe could be used,after dilator or dilators have been used, to further explore tissue thatmay still be in the surgical field despite use of the dilator(s). Groundelectrode 145 could be located proximal to electrodes 105, 110, 115,120, 125, 130, 135, and 140 or off device 100 entirely. Also,embodiments are not limited to LIF but may be implemented treatingvarious disorders with procedures (e.g., endoscopic procedures), relatedor unrelated to spinal therapy, that would benefit from nerve mapping.

Embodiments may be implemented in many different system types. Referringnow to FIG. 6, shown is a block diagram of a system in accordance withan embodiment of the present invention. Multiprocessor system 700 is apoint-to-point (P-P) interconnect system and includes first processor770 and second processor 780 coupled via point-to-point interconnect750. Each of processors 770 and 780 may be multicore processors. Theterm “processor” may refer to any device or portion of a device thatprocesses electronic data from registers and/or memory to transform thatelectronic data into other electronic data that may be stored inregisters and/or memory. First processor 770 may include a memorycontroller hub (MCH) and P-P interfaces. Similarly, second processor 780may include a MCH and P-P interfaces. The MCHs may couple the processorsto respective memories, namely memory 732 and memory 734, which may beportions of main memory (e.g., dynamic random access memory (DRAM))locally attached to the respective processors. First processor 770 andsecond processor 780 may be coupled to chipset 790 via P-P interconnects752, 754 respectively. Furthermore, chipset 790 may be coupled to firstbus 716 via an interface. Various input/output (I/O) devices 714 may becoupled to first bus 716, along with bus bridge 718, which couples firstbus 716 to second bus 720. Various devices may be coupled to second bus720 including, for example, keyboard/mouse 722, communication devices726, and data storage unit 728 (e.g., disk drive or other mass storagedevice that includes code 730). Further, audio I/O 724 may be coupled tosecond bus 720.

Embodiments may be implemented in code and may be stored on a storagemedium having stored thereon instructions which can be used to program asystem to perform the instructions. The storage medium may benon-transitory and may include, but is not limited to, any type of diskincluding floppy disks, optical disks, solid state drives (SSDs),compact disk read-only memories (CD-ROMs), compact disk rewritables(CD-RWs), and magneto-optical disks, semiconductor devices such asread-only memories (ROMs), random access memories (RAMs) such as DRAMs,static random access memories (SRAMs), erasable programmable read-onlymemories (EPROMs), flash memories, electrically erasable programmableread-only memories (EEPROMs), magnetic or optical cards, or any othertype of media suitable for storing electronic instructions. Suchcomputer-readable or computer-usable storage medium or media is (are)considered to be part of an article (or article of manufacture). Anarticle or article of manufacture can refer to any manufactured singlecomponent or multiple components.

Embodiments of the invention may be described herein with reference todata such as instructions, functions, procedures, data structures,application programs, configuration settings, code, and the like. Whenthe data is accessed by a machine, the machine may respond by performingtasks, defining abstract data types, establishing low-level hardwarecontexts, and/or performing other operations, as described in greaterdetail herein. The data may be stored in volatile and/or non-volatiledata storage. The terms “code” or “program” cover a broad range ofcomponents and constructs, including applications, drivers, processes,routines, methods, modules, and subprograms. Thus, the terms “code” or“program” may be used to refer to any collection of instructions which,when executed by a processing system, performs a desired operation oroperations. In addition, alternative embodiments may include processes(in code or otherwise) that use fewer than all of the disclosedoperations, processes that use additional operations, processes that usethe same operations in a different sequence, and processes in which theindividual operations disclosed herein are combined, subdivided, orotherwise altered.

While the present invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations as fall within thetrue spirit and scope of this present invention.

What is claimed is:
 1. A system to monitor nerve tissue comprising: asurgical instrument including a distal end portion coupled to first,second, third, and fourth electrodes; a memory; a processor, coupled tothe memory, configured to cause a current source unit to: (a)simultaneously communicate a first current application to the firstelectrode, a second current application to the second electrode, a thirdcurrent application to the third electrode, and a fourth currentapplication to the fourth electrode; (b) sense a first evoked responseto any of the first, second, third, and fourth current applications ifthe first evoked response occurs; (c) when no evoked response to thefirst, second, third, and fourth current applications is sensed,determine a first nerve proximity state is present; (d) when the firstevoked response is sensed, non-simultaneously titrate additional firstcurrent to the first electrode and additional second current to thesecond electrode and sense additional evoked responses, if they areproduced, until an evoked response is sensed for only one of the firstand second electrodes in response to equal amplitudes of currentapplication; (e) after step (d), simultaneously communicate asupplemental first current application to the first electrode, asupplemental second current application to the second electrode, asupplemental third current application to the third electrode, and asupplemental fourth current application to the fourth electrode; (f)sense a supplemental first evoked response to any of the supplementalfirst, second, third, and fourth current applications if thesupplemental first evoked response occurs; (g) when no evoked responseto the supplemental first, second, third, and fourth currentapplications is sensed, determine the first nerve proximity state ispresent; (h) when the supplemental first evoked response is sensed,non-simultaneously titrate additional third current to the thirdelectrode and additional fourth current to the fourth electrode andsense supplemental evoked responses until a supplemental evoked responseis sensed for only one of the third and fourth electrodes in response toequal amplitudes of current application; (i) communicate to a user ofthe surgical instrument a second nerve proximity state is presentindicating a nerve is closer to one of the first, second, third, andfourth electrodes than another of the first, second, third, and fourthelectrodes in response to at least one of steps (d) and (h); whereintitrating the additional first, second, third, and fourth currentsincludes incrementally changing the additional first, second, third, andfourth currents.
 2. The system of claim 1, wherein the memory includesat least one memory and the processor includes at least one processor.3. The system of claim 2 comprising the current source unit, whichincludes a current source, an amplifier, a filter, an A/D converter, aD/A converter, and a multiprocessor system, wherein the multiprocessorsystem includes the memory and the processor.
 4. The system of claim 2,wherein the processor is configured to cause the current source unit to,when the first evoked response is sensed, titrate the additional firstcurrent to the first electrode and the additional second current to thesecond electrode, without supplying additional current to the third andfourth electrodes, and sense additional evoked responses if they areproduced, until an evoked response is sensed for only one of the firstand second electrodes in response to equal amplitudes of currentapplication.
 5. The system of claim 1 wherein the surgical instrumentincludes at least one of a probe, a dilator, a retractor, snips, acutting device, a rasp, a trocar, spreaders, distracters, a shim, ascraper, a chisel, disc cutters, a curette, a suction probe, or tamps.6. An article comprising a non-transitory medium storing instructionsthat enable a processor-based system to: (a) simultaneously communicate,via a current source unit, a first current application to a firstelectrode, a second current application to a second electrode, a thirdcurrent application to a third electrode, and a fourth currentapplication to a fourth electrode; wherein the first, second, third, andfourth electrodes are coupled to a surgical instrument; (b) sense afirst evoked response to any of the first, second, third, and fourthcurrent applications if the first evoked response occurs; (c) when noevoked response to the first, second, third, and fourth currentapplications is sensed, determine a first nerve proximity state ispresent; (d) when the first evoked response is sensed,non-simultaneously titrate additional first current to the firstelectrode and additional second current to the second electrode andsense additional evoked responses until an evoked response is sensed foronly one of the first and second electrodes in response to equalamplitudes of current application; (e) after step (d), simultaneouslycommunicate a supplemental first current application to the firstelectrode, a supplemental second current application to the secondelectrode, a supplemental third current application to the thirdelectrode, and a supplemental fourth current application to the fourthelectrode; (f) sense a supplemental first evoked response to any of thesupplemental first, second, third, and fourth current applications ifthe supplemental first evoked response occurs; (g) when no evokedresponse to the supplemental first, second, third, and fourth current issensed, determine the first nerve proximity state is present; (h) whenthe supplemental first evoked response is sensed, non-simultaneouslytitrate additional third current to the third electrode and additionalfourth current to the fourth electrode and sense supplemental evokedresponses until a supplemental evoked response is sensed for only one ofthe third and fourth electrodes in response to equal amplitudes ofcurrent application; (i) communicate to a user of the surgicalinstrument a second nerve proximity state is present indicating a nerveis closer to one of the first, second, third, and fourth electrodes thananother of the first, second, third, and fourth electrodes in responseto at least one of steps (d) and (h); wherein titrating the additionalfirst, second, third, and fourth currents includes incrementallychanging the additional first, second, third, and fourth currents. 7.The article of claim 6 storing instructions that enable the system to:simultaneously communicate current to the first and second electrodeswhile communicating no current to the third and fourth electrodes; sensean evoked response to the simultaneously communicated current if theevoked response occurs; when no evoked response to the simultaneouslycommunicated current is sensed, simultaneously communicate current tothe third and fourth electrodes while communicating no current to thefirst and second electrodes.
 8. The article of claim 6, wherein:communicating the first current application to the first electrodecomprises communicating bipolar current between the first electrode anda reference electrode; the first, second, third, fourth, and referenceelectrodes are all included on a distal end portion of the surgicalinstrument.
 9. The article of claim 6, wherein the surgical instrumentincludes at least one of a probe, a dilator, a retractor, snips, acutting device, a rasp, a trocar, spreaders, distracters, a shim, ascraper, a chisel, disc cutters, a curette, a suction probe, or tamps.10. An article comprising a non-transitory medium storing instructionsthat enable a processor-based system to: (a) simultaneously communicate,via a current source unit, a first current application to a firstelectrode, a second current application to a second electrode, a thirdcurrent application to a third electrode, and a fourth currentapplication to a fourth electrode; wherein the first, second, third, andfourth electrodes are coupled to a surgical instrument; (b) sense afirst evoked response to any of the first, second, third, and fourthcurrent applications if the first evoked response occurs; (c) when noevoked response to the first, second, third, and fourth currentapplications is sensed, determine a first nerve proximity state ispresent; (d) when the first evoked response is sensed,non-simultaneously titrate additional first current to the firstelectrode and additional second current to the second electrode andsense additional evoked responses until an evoked response is sensed foronly one of the first and second electrodes in response to equalamplitudes of current application; (e) communicate to a user of thesurgical instrument a second nerve proximity state is present indicatinga nerve is closer to one of the first, second, third, and fourthelectrodes than another of the first, second, third, and fourthelectrodes in response to step (d); wherein titrating the additionalfirst and second currents includes incrementally changing the additionalfirst and second currents.
 11. The article of claim 10 storinginstructions that enable the system to: simultaneously communicatecurrent to the first and second electrodes while communicating nocurrent to the third and fourth electrodes; sense an evoked response tothe simultaneously communicated current if the evoked response occurs;when no evoked response to the simultaneously communicated current issensed, simultaneously communicate current to the third and fourthelectrodes while communicating no current to the first and secondelectrodes.
 12. The article of claim 10, wherein: communicating thefirst current application to the first electrode comprises communicatingbipolar current between the first electrode and a reference electrode;the first, second, third, fourth, and reference electrodes are allincluded on a distal end portion of the surgical instrument.
 13. Thearticle of claim 10, wherein the surgical instrument includes at leastone of a probe, a dilator, a retractor, snips, a cutting device, a rasp,a trocar, spreaders, distracters, a shim, a scraper, a chisel, disccutters, a curette, a suction probe, or tamps.